The present invention is in a process of removing zinc from a zinc containing sludge formed in a phosphating process. The process of the invention is conducted at elevated temperatures and under reducing conditions in the presence of additives. The invention also includes the use of the process in the Waelz processing of metallurgical residual dusts.
Phosphating processes using zinc phosphate solutions are widely used in the metal-working industry on a large variety of metal surfaces. The phosphate layers formed on metals by that process serve particularly to facilitate sliding and non-cutting cold working, for protection against corrosion, and for improving the adherence of paint. In a phosphating process the desired conversion layer is formed on the metal surface. However, a phosphating process sludge, which must be removed periodically from the treating baths, is also formed. In a dried state, the sludge formed during the treatment of steel and iron has the following composition: 2 to 20% Zn; 10 to 30% Fe; 30 to 45% P.sub.2 O.sub.5. In the phosphating of zinc surfaces, the sludge will contain about 38 to 45% Zn. In addition to the foregoing main components, the sludge, depending on the specific phosphating bath composition, may also contain small amounts of Na, Mn, Ni, Ca, Cu, Co and other cations. The sludge may also contain small amounts of anions, such as NO.sub.3-, Cl.sup.-, and F.sup.- .
The phosphating process sludge settles in the phosphating bath and has a residual moisture content of 60 to 95%. That residual moisture content can be decreased to 30 to 50% if the sludge is squeezed out. The sludge can be dried to a moisture content of less than 10%.
The formation of sludge in the phosphating process results in disadvantages. Two major disadvantages are that the sludge binds a portion of the bath chemicals and the sludge must be disposed of as hazardous waste because of its heavy metal content. For this reason, there have been efforts to develop a process to recover the valuable substances from the sludge and to decrease the amount of waste to be disposed.
For instance, DE-A-2629776 describes a wet chemical process of treating the phosphating process sludge. That process has not yet been commercially accepted because of its economics and difficult processing technology.
U.S. Pat. No. 3,653,875 proposes treating the sludge with an alkali and reducing the solids thus obtained at 500.degree. to 1300.degree. C. to recover zinc. Trisodium phosphate can be washed out of the residue. That process also has not been commercially accepted because it is expensive as it requires a plurality of steps.
It is an object cf the invention to provide a process by which the valuable substances phosphorus and zinc contained in the phosphating process sludge are transformed into a usable form.
It is another object of the invention to provide such a process which does not result in waste waters.
It is a further object of the invention to provide a process which can be carried out in a particularly simple manner